Remarks on the Current State of Adsorption Kinetic Theories for Heterogeneous Solid Surfaces: A Comparison of the ART and the SRT Approaches

At the end of the 1980s, the classical theories of adsorption/desorption kinetics based on ideas of the ART (absolute rate theory) were challenged by new theories linking the rate of adsorption/desorption with the chemical potentials of free (bulk) and adsorbed molecules. Among the latter theories, the so-called statistical rate theory (SRT) received probably the most advanced theoretical development. It is based on quantum mechanics and statistical thermodynamics. The appearance of the new theories sometimes gives rise to emotional discussions in the literature, but they do not always focus on fundamentals. The present paper is aimed at comparison of the theoretical predictions of the ART and the SRT approaches with the behavior of adsorption/desorption kinetics in real adsorption systems. That comparison is based on accepting the popular Langmuir model of adsorption, next generalized for the case of energetically heterogeneous solid surfaces, typical for real adsorption systems. For the purposes of comparison, the kinetics of CO 2 adsorption on scandia has been subjected to quantitative analysis using the expressions offered by both the classical ART approach and the newer SRT approach. For a certain set of parameters, the SRT theoretical expression reproduces very well the behavior of both kinetic and equilibrium adsorption isotherms. Also, the determined parameters exhibit a fully physical meaning. In the case of the ART approach, one can fit the kinetic adsorption isotherms, using many sets of parameters, but some of the determined parameters always exhibit a nonphysical meaning.